Recently, polymer electrolyte membrane (PEM) fuel cells have generated significant interest for automotive applications. Automotive competitiveness requires a catalyst-related cell voltage loss less than about 50 mV over the entire current density range over an automotive lifetime greater than 10 years. This may include about 300,000 large load cycles and about 30,000 start/stop cycles. Such durability goals have been limited by cathode electrode degradation, in particular due to carbon corrosion. Factors that impact carbon corrosion include the many startup/shutdown cycles and localized H2 starvation. Startup/shutdown associated carbon corrosion may be due to a local high potential between parts of the cathode and the adjacent electrolyte while an air/H2 front is passing through the anode when the cathode is still filled with air. This local high potential drives rapid carbon corrosion.
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen-rich gas or pure hydrogen and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work may be used to operate a vehicle, for example.
PEM fuel cells are popular for vehicle applications. The PEM fuel cell generally includes a solid-polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture, and the membrane defines a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack includes a series of flow field or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. The fuel cell stack receives an anode hydrogen reactant gas that flows into the anode side of the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of the MEA. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of the MEA. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include liquid water as a stack by-product. The bipolar plates may also include flow channels for a cooling fluid. An automotive fuel cell stack, for example, may include about two hundred or more bipolar plates.
The bipolar plates are typically made of a conductive material, such as stainless steel, titanium, aluminum, polymeric carbon composites, or graphite etc., so that they conduct the electricity generated by the fuel cells from one cell to the next cell and out of the stack. Metal bipolar plates typically produce a natural oxide on their outer surface that makes them resistant to corrosion. However, this oxide layer is not conductive, and thus increases the internal resistance of the fuel cell, reducing its electrical performance. Also, the oxide layer may frequently make the plates more hydrophobic.